1. Field of the Invention
The present invention relates to a method for control channel transmission in a wireless communication system, and more specifically, a method for coding and mapping control channel information in a wireless communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a popular wireless communication technology to multiplex data in frequency domain.
In a communication system, a multi-path communication channel results in a frequency-selective fading. Moreover, in a mobile wireless environment, the channel also results in a time-varying fading. Therefore, in a wireless mobile system employing OFDM, the overall system performance and efficiency can be improved by using, in addition to time-domain scheduling, frequency-selective multi-user scheduling. In case of frequency-selective multi-user scheduling, a contiguous set of subcarriers potentially experiencing an upfade is allocated for transmission to a user. Upfade is a situation where multipath conditions cause a radio signal to gain strength. The total bandwidth is divided into multiple subbands, and each subband contains multiple contiguous subcarriers.
A multiple antenna communication system, which is often referred to as multiple input multiple output (MIMO) system, is widely used in combination with OFDM technology, in a wireless communication system to improve system performance.
In a MIMO system, both transmitter and receiver are equipped with multiple antennas. Therefore, the transmitter is capable of transmitting independent data streams simultaneously in the same frequency band. Unlike traditional means of increasing throughput (i.e., the amount of data transmitted per time unit) by increasing bandwidth or increasing overall transmit power, MIMO technology increases the spectral efficiency of a wireless communication system by exploiting the additional dimension of freedom in the space domain due to multiple antennas. Therefore MIMO technology can significantly increase the throughput and range of the system.
When the transmission channels between the transmitters and the receivers are relatively constant, it is possible to use a closed-loop MIMO scheme to further improve system performance. In a closed-loop MIMO system, the receiver informs the transmitter of feedback information regarding the channel condition. The transmitter utilizes this feedback information, together with other considerations such as scheduling priority, data and resource availability, to optimize the transmission scheme.
A popular closed-loop MIMO scheme is MIMO preceding. With preceding, the data streams to be transmitted are precoded, i.e., pre-multiplied by a precoding matrix, before being passed on to the multiple transmit antennas in a transmitter.
In a contemporary closed-loop MIMO precoding scheme, when a transmitter precodes data before transmitting the data to a receiver, the transmitter informs the receiver of the precoding information such as an identification of the precoding matrix by transmitting explicit control information that carries the precoding information. A significant problem with this approach is that the control information inefficiently consumes a significant amount of system resources and degrades the overall system throughput and capacity.
In packet-based wireless data communication systems, a so-called control channel usually accompanies the data transmission. In the third Generation Long Term Evolution (3G LTE) system, the control channel that carries the control signal is referred to as Physical Downlink Control Channel (PDCCH) for transmission from a base station to a unit of user equipment, or Physical Uplink Control Channel (PUCCH) for transmission from a unit of user equipment to a base station. The PDCCH carries information such as user equipment (UE) ID, resource assignment information, Payload size, modulation, Hybrid Automatic Repeat-reQuest (ARQ) HARQ information, MIMO related information. A Cyclic Redundancy check calculated over the control information and masked by the UE ID can be carrier instead of explicit UE ID.
The different types of feedback information from the UE carried in PUCCH are summarized below:
Subbands CQI Information
MIMO Rank
Antenna/Layer selection
MIMO Precoding
ACK/NACK for downlink data transmission
Contemporarily, each type of feedback control information is separately coded or modulated and transmitted. This results in inefficient transmission because efficient coding across multiple control types cannot be employed. Moreover, if some type of CRC is used for error detection purposes, separate CRCs are required for each of the control information types resulting in excessive overhead.